Scanning electron microscope wherein an image is formed as a function of specimen current



United States Patentf [72] inventors William G. Morris [56] ReferencesCited gcfillcen m;mb N Y UNITED STATES PATENTS L 3 337 729 8/1967 Thomasetal. 250/49.5(8) 765,323 3341'? 4 9 1967 Th t 1. 50 49.5 s 221 FiledOct-7,1968 a 2 [45] Patented Dec,22, 1970 Pnr nary Examiner-JamesLawrence [73] Assignee General Electri Com AsststantExarnmer-A. L.B1rcha corporation f Ne Y k Attorneys-Richard R. Brainard, Marvin Snyder,Paul A.

Frank, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg[54] SCAM ING ELECTRON MICROSCOPE WHEREIN ABSTRACT: A system forproducing high resolution mag- AN IMAGE IS FORMED AS A FUNCTION OFSPECIMEN CURRENT 9 Claims, 12 Drawing Figs.

US. Cl. 250/495, l78/7.5 Int. Cl. H0lj 37/26 Field of Search 250/49.5( l49.5(8), 49.5(5)

4MP. C0007!!! nified images of a specimen includes an electron beamscanning in a two-dimensional pattern over the surface of the specimen,and modulates brightness of an electromagnetically deflected cathode raytube according to a signal proportional to instantaneous specimencurrent. The voltage drop across a resistor connected to the specimencomprises the specimen signal. A crystal oscillator synchronizesscanning of the electron beam to scanning of the cathode ray tube.

. l SCANNING ELECTRON MICROSCOPE WIIEREIN AN IMAGE IS FORMED AS AFUNCTION OF SPECIMEN CURRENT INTRODUCTION This invention relates toelectron microscopy, and more particularly to a system in which anelectron beam scans a specimen so as to generate magnified imagesthereof.

Scanning electron microscopes have proven to be a valuable aid inexamining specimens microscopically. Typically, a scanning electronmicroscopeemploys a finely focused beam of electrons which is scannedover the surface of the specimen. Electrons which are eitherbackscattered from the primary beam by the specimen or given off by thespecimen as secondary emission electrons are detected, generating asignal proportional to the rate at which the electrons are detected.This signal is then used to modulate brightness on a cathode ray tubewhich scans in synchronism with the primary electron beam, so that amagnified image of the specimen is formed on the face of the cathode raytube. The magnification thus obtained is a ratio of any linear distancescanned on the specimen to the corresponding linear distance scanned onthe face of the cathode ray tube.

l-Ieretofore, scanningelectron microscopes have been relatively complex.This complexity has been due, in large measure, to need for properplacement and orientation of electron detectors to detect either therelatively high energy primary backscattered electrons or the relativelylow energy secondary emission electrons with high efficiency and withoutintroduction of appreciable noise in'the detector output signal. Theseprior scanning electron microscopes have also been subject to imageaberrations due to power line frequency magnetic fields. Moreover,brightness, spot size and contrast on the cathode ray tube face,which'depend on cathode ray tube anode voltage amplitude have been lessthan satisfactory since the electrostatic deflection and focusingemployed in the cathode ray tube to achieve precise and lineardeflection of the cathode ray oreleetron beam therein, impose relativelylow maximum limitations on cathode ray tube anode voltage amplitude.

The present invention concerns a scanning electron microscope which hasbeen simplified by avoiding any need for electron detectors and theirassociated placement and orientation requirements. Instead of detectingprimary backscattered electrons or secondary emission electrons, theapparatus of the instant invention responds to specimen current, orelectron flow through the specimen, by sensing current fiow through acircuit connected in series with the specimen. Moreover, by employingelecu'omagnetic deflection and focusing in the cathode ray tube, cathoderay tube anode voltage is increased, resulting in improved brightness,spot size and contrast in the displayed image. Aberrations in thedisplayed image due to interference from the power line frequencymagnetic fields are minimized by employing a crystal oscillator togenerate the basic timing signals for the system.

Accordingly, one object of the invention is to provide a simplifiedscanning electron microscope wherein the image of an observed specimenis determined in accordance with sensed specimen current.

Another object is to I provide a scanning electron microscope withimproved brightness, spot size and contrast in the displayed image.

Another object is to provide a scanning electron microscope whereinaberrations due to power line frequency magnetic fields are minimized.

Briefly, in accordance with a preferred embodiment of the invention, asystem for producing high resolution magnified images of a specimen witha scanning electron beam comprises magnetic field generating means forsweeping the elec-. tron beam in a predetermined two-dimensional patternover the surface of the specimen undergoing observation and meanscontrolling the rate attwhich the electron beam is swept BRIEFDESCRIPTION OF THE DRAWINGS The features of the invention believed to benovel are set forth with particularity in the appended claims. Theinvention itself, however, both as to organization and method ofoperation, together with further objects and advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings in which:

FIG. 1 is a block diagram of a system embodying the instant invention;

FIGS. 2A2I-I are waveforms of output signals produced by designatedapparatus in the system shown in FIG. 1;

FIG. 3 is a schematic diagram of a high-gain low-noise specimen currentamplifier which may be employed in the system shown in FIG. 1;

FIG. 4 is a schematic diagram of a constant current amplifier fordriving magnetic deflection coils which may be employed in the systemshown in FIG. 1; and

FIG. 5 is a schematic diagam of a tiltable specimen holder which may beemployed in the system shown in FIG. 1.

DESCRIPTION OF TYPICAL EMBODIMENTS In FIG. 1, a crystal oscillator 10 isshown driving a decimal counter 12 through an amplifier 11. Oscillator10 may be operated at any convenient frequency, such as 31,500 Hz, forexample, and counter 12 may be set to count to any predeterminedmultiple or submultiple of the: power line frequency. Typicalsubmultiples and multiples of the power line frequency to which counter12 may be set are 10, 30 and 300, for example. Each time counter 12 hascounted to a selected predetermined multiple .or submultiple of thepower line frequency, a horizontal retrace pulse generator 13 isactuated to furnish a resetting pulse jointly to a horizontal sweepgenerator 14 and the first input of a 2-input OR gate 45. Horizontalsweep generator 14 comprises an integrating circuit which integrates aconstant current supplied for a constant current generator '15, andhence provides a ramp type voltage; that is, a voltage of substantiallyconstant slope is generated upon occurrence of each horizontal retracepulse from pulse generator 13. The amplitude of current supplied byconstant current generator 15 is adjusted each time the preset count ofcounter 12 is changed by virtue of a mechanical interconnection 9. Thus,constant current generator 15 provides facility for adjusting the slopeof output voltage from horizontal sweep generator 14. This slopingvoltage causes the electron beam in both an electron optical column 21and a cathode ray tube 31 to sweep horizontally in a smooth manner.

Output signals from horizontal sweep generator 14 are furnished jointlyto a pair of constant current amplifiers 17 and 18. Constant currentamplifier l7 drives horizontal scanning coils 20 in column 21, whileconstant current amplifier 18 furnishes input signals to horizontalscanning coils 46 of cathode ray tube 31.

Output signals from counter 12 are also furnished to a digital counter32, which functions as a frequency divider so as to produce a singleoutput pulse each time a predetermined number of input pulses have beencounted. Counter 32 may conveniently be set to count 128, 256,. 512 or1,024 input pulses. Each time counter 32 has counted to itspredetermined number of input pulses, a vertical retrace pulse generator33 is actuated to-furnish a resetting pulse jointly to a vertical sweepgenerator 34 and the second input of 2-input OR gate 45.

Vertical sweep generator 34 provides a staircase type voltage; that is,each pulse produced by a vertical trace pulse generator 37 is integratedby vertical sweep generator 34 in typical staircase counter fashion,causing the electron beam in electron optical column 21 and the cathoderay in cathode ray predetermined number of vertical trace pulses havebeen integrated by vertical sweep generator 34, counter 32 furnishes an'outputsignal to vertical'retrace pulse generator 33 which thereuponresets itself to its initial output amplitude. This returns the electronbeam and the cathode ray to their initial vertical positions preliminaryto resuming another cycle of vertical stepwise advancement.

For operation in which the electron beam inelectron optical column 21 isto sweep across a predetermined horizontal distance, the setting ofconstant current generator adjusts the slope of output voltage producedby horizontal sweep generator 14so that the requisite amplitude forcompleting each horizontal sweep is reached simultaneously withinitiation of the next vertical trace pulse. Thus, each horizontalretrace pulse returns the' beam in a generally horizontal direction toits-initial horizontal starting point. Simultaneously, the staircasesignal from vertical sweep generator 34 is furnished to the input ofeach one of a pair of constant current amplifiers 35 and 39. Constantcurrent amplifier 35 drives magnetic deflection coils 22 in electronoptical column 21 so .as to move the electron beam in a verticaldirection, while signal made up of electronsleaving specimen '23isfurnished to a high gain, low" noise, specimen current amplifier 38directly from specimen 23. Output signals from specimen currentamplifier 38 are furnished to the'signal input of a gated videoamplifier 40. Energization of the gating or control input of amplifier40, which thereupon prevents or inhibits any output signal fromamplifier 40, is furnished by OR gate 45. Input signals to the intensitycontrol input or grid 48 of cathode ray tube 31 are supplied by-gatedvideo amplifier 40 in absence of an output signal from OR gate 45.However, when either the first or second input to OR gate 45 isfulfilled, gated video amplifier 40 is switched to its'nonconductive orblocking condition, preventing the output signal of specimen currentamplifier 38 from reaching the intensity control input of cathode raytube 31. Thus, gated. video amplifier40 facilitates blanking, hereindefined as interrupting the cathode ray (or electron beam) incathode raytube 31 during those times in which the cathode ray is not forming apartof the image being displayed on the face' of the cathode raytube.

Within electron optical column 21, the electron beam 24 (which assumesthe outline indicated by dotted lines) is furnished from an electron gun25 through a beam-defining apertured anode 26. The beam is focused bymeans of first and second magnetic condenser lenses 27 and 28,comprising electromagnetic coils energized with constant voltage from DCbias sources 41 and 42 respectively. Bias sources 41 and 42 may beadjusted independently to desired amplitude values for focusing theelectron beam within electron optical column 21. In addition, a magneticobjective lens 29 is likewise situated in electron optical column 21,and comprises a coil energized with a constant voltage from a DC biassource 43. DC bias source 43 may be adjusted to provide to coils 29 theamplitude of voltage a required tofocus the electron beam on specimen23. The electron beam in electron optical column 21 finally passesthrough a beam defining apertured plate 19 before impinging uponspecimen 23. A vacuum is maintained within electron optical column 21 bypumps (not shown) which exhaust the column through a port 44 in theelectrically grounded walls of the column.

In considering operation of the system of FIG. I, assume that horizontalsweep generator 14 and vertical sweep generator 34 are each in theirinitial output conditions, and that the electron beam in electronoptical column 21 is finely focused by virtue of proper amplitudesettings on DC bias means 41, 42 and 43. Pulses produced by crystaloscillator 10 and illustrated in FIG. 2A, are .counted by counter 12.Counter 12 produces an output pulse each time a preset number of inputpulses have been counted, thereby acting as a frequency divider. Eachpulse suppliedby counter 12, such as illustrated in FIG. 2B, driveshorizontal retrace pulse generator 13 which produces horizontal retracepulses such as illustrated in FIG.

2C. Each horizontal retrace pulse resets horizontal sweep generator 14to its initial condition and, by fulfilling the first input to OR gate45, gates video amplifier 40 into its nonconductive condition throughoutthe horizontal retrace pulse duration so as to prevent energization ofgrid 48 of cathode ray tube 31 with signals from specimen currentamplifier 38. Blanking of cathode ray-tube 31 is thus achieved,preventing distortion of the image on the face of cathode ray tube 31from occurring while the horizontal retrace pulse endures.

Upon completion of each horizontal retrace pulse, a ramp voltage isproduced by horizontal sweep generator 14 which integrates chargesupplied by constant current generator 15. The amplitude of currentsupplied by constant current generator 15 is selected through linkage 9whenever a count is preset in counter 12. Horizontal sweep generator 14continues to integrate charge until the next horizontal retrace pulseoccurs returning the horizontalsweep generator output'voltage to zero.The resulting output voltage waveform produced by horizontal sweepgenerator 14 is illustrated in FIG. 2D.

Each pulse from counter, 12 which actuates horizontal retrace pulsegenerator 13 to, reset horizontal sweep generator 14 to its initialcondition also-actuates vertical-trace pulse generator 37 to advance theoutput voltage amplitude of vertical sweep generator 34by a singleuniform increment. The size of this increment is determined by thevertical trace pulse amplitude which is selected through linkage 8 whenthe number of horizontal lines to be generated in each raster isselected. This selection is made by setting counter 32 to producean'output pulse after counting a predetermined number of input pulses.The output signal of vertical trace pulse generator 37 is illustrated inFIG. 2B, while the incrementally increasing amplitude voltage producedby vertical sweep generator 34 is illustrated in FIG. 2F.

When counter 32 has counted to its preset count, an output pulse, suchas illustrated in FIG. 26, is supplied to vertical retrace pulsegenerator 33, resulting in a vertical retrace pulse such as illustratedin FIG. 2H. This vertical retrace pulse resets horizontalpositionsosimilarly, a vertical signal is generated As theelectronelectron optical column 21 is I scanned across the surface of specimen23, current leaving the specimen flows to low input impedance specimencurrent amplifier 38. The low inputirnpedance of amplifier 38-helpsprevent deleterious electric fields from forming on specimen 23. Theamplified specimen current is furnished by amplifier 38 throughgated video amplifier. 40 to. grid 48 of cathode ray tube 31. Thus,'the image onthe face of cathode ray tube 31 is modulatedin accordance with amplitudeof specimen current from specimen-23. For a constant electron beamcurrent in too irregular to be observed "in. a conventional lightmicroscope. The scanning electron microscope of the instant inventionalso superior to conventional electron microscopes in thestudyof veryfragile surface structures which cannot be replicated; infact,-nospecimen preparation is ordinarily required, except for cuttingto a convenient size and mountingon the specimen holder. If thematerialto be examined exceedsl ohm-centimeters in resistivity, a thin layer ofcarbon or aluminum evaporated over the surface of the specimen usuallyovercomes any static voltage built up on the specimen which mightotherwise result in dielectric breakdown. This also avoids degradationin image resolution due to the varying surfacepotential'which wouldotherwise result on aspecimenofthistype; I f. i

The resolution obtainable in a scanned image is equalto the size ofthcelectron beam-striking thespecimen, to a first ap proximation. As thebeam size is reduced by the electron len sea in the electron opticalcolumn, the current in the beam is also reduced, as well as the ratio ofsignal to noise. Therefore, it is'highly desirable to employ a low noisepreamplifier in the first stage of the video signal amplifying apparatussystem.;A preamplifier of this type is illustrated in FIG. 3. v

Thus, in FIG. 3,-specimen current amplifier 38 is illustrated tocomprise an operational amplifier 50 which permits amplification ofintensity or videosignals as'small as am- Peres without introducingobjectionable noise. A feedback resistance 51 determi es thegain-obtained from operational amplifier 50 while a capacitance 52 inshunt with resistance 51 is usedto reduce the high-frequency gain of thecircuit and prevent oscillation. A .pair of low-leakage diodes-53am 54,connected in "opposed parallel relationship, protect amplifier 50 bypreventing voltages at its input from deviating more than about 1 voltfrom ground potential. Since the specimen signal is derived from a highvoltage electron beam which can have large voltage transients, diodes 53and 54 prevent damage to thesensitive field effect transistor input ofoperational amplifier 50 which might otherwise result. The low impedanceinput to operational amplifier .50, which results from the use ofnegative feedback through resistance 51, permits employment of severalfeet of coaxial cable between the specimen and amplifier 50 without anyadverse effect upon frequency response of the apparatus.

The second stage of specimen current amplifier 38 comprises adifferential input operational amplifier 55 having a feedback resistance56 coupling a first input of operational amplifier 55 to the outputthereof. An additional resistance 57 is connected between a second inputto operational amplifier 55 and ground. A double-pole-doublethrow imagepolarity switch 58 is used for reversing output signal polarity ofspecimen current amplifier 38 so that positive or negative images may bedisplayed. Switch 58 is connected to the first .r ,15] V. ing upon theposition or mien ss, the output of amplifier 50 is coupled to the firstinput of amplifier 55 and a positive bias is coupled to the second inputof amplifier 55, as

shown in FIG. 3, or the'positive bias is coupled to the first input ofoperational arnplifier-55 and the output of amplifier 50 is coupled totheseoond'input of operational amplifier 55. Therefore, by changingposition .of switch 58, the polarity of output signal from amplifier 38is'reversed, permitting display of either positive or negative The biaslevel control comprising potentiometer 62- and resistance-63 permits thesteady-background of the specimen current to be subtracted out of thesignal, so that only the useful video'signal is amplified by the secondstage of specimen current amplifier 38.

By employing magnetic focusing and deflection of the cathode ray in thecathode ray tube, the quality of display is further improved.Brightness, spot size and contrast improve with higher anode voltages onthe'cathode ray tube, but electrostatic deflection and focusing of .thebeam. become more difiicult under these circumstances. Therefore, inorder to make the magnetic deflection system precise and linear over awide range of sweepfrequencies, magnetic deflection ofthe electron beamin both the cathode ray tube and the electron optical column isemployed. The same type .of circuit is employed in both cases,'andisillustrated'in FIG. 4. An operational amplifier 70 receiving inputsignals through a currentlimiting resistance 74 drives magneticdeflection coils'7l, with feedback voltage obtained across a resistance72 .and

furnished to the input of amplifienillthrough a feedbackreoperation.Hence, the current flowing through windings 71 is proportional to thevoltage applied to the input of amplifier 70 sistance 73 in order torealize a 'constant current mode of within the frequency response limitsof the amplifier. Magnifiand second inputs of operational amplifier 55through input cation achieved in the image can be increased by reducingcurrent in the deflection coils of the electron optical column. This isaccomplished by increasing the value of resistance 72, thereby reducingthe gain in this amplifier stage.

Many specimen structures have'deep relief and hence are best studiedstereoscopically. The present invention facilitates making stereoscopicpairs of micrographs, or images of mic roscopic regions, by employing, atilting specimen holder '80 within electron column 21, as illustratedschematically in FIG. 5. Specimen holder comprises a bistable platform,such as of nickel, to which isspot-weldedfla wire rodfulcrumi8l for theplatform. The diameter of rod 81 is chosenin order to provide aparticular desired stereo-angle. Wire rod 81 rotates atop a nonmagnetic,electrically conductive support 82 to which the specimen currentamplifier input is connected. The electron beam impinging on specimen23is indicatedbythe vertical'arrow. 9 t x A pair of electromagnets 83and'84 are situated beneath either end of support 82, so as to exert amagnetic influence on bistable platform 80. Electromagnets 83 and 84 areenergized from a power supply 85 through a pair'of pushbuttons 86 and87'respectively. Thus, during viewing, either of the two bistablepositions of platform80 can be selected by. momentarily depressing theappropriate one of pushbuttons-8 6 and 87, momentarily activating theappropriate electromagnet. This selects one or the other of two possibleangles of incidence for theelectron beam. Upon Ideenergization of thepreviously energized electromagnet, the platform remains in in selectedposition. Stereoscopic pairs of micrographs may then be made byphotographing the displayed-image of a particular area or feature twiceon specimen. 23, once during each of the two bistable positions assumedby the platform.

The foregoing describes a simplified scanning electron microscopewherein the image of tin observed specimen is determined in accordancewith sensed specimen current. The

microscope provides improved brightness, spot size and contrast in thedisplayed imagegln additiomirnage aberrations .due to interference frompower. line frequency magnetic fields are minimized.

While only certain preferred features of the invention havepotentiometer 62 compriseabiaslevel control. Thus, depend- 75 been shownby way of illustration, many modifications and changes will occur tothose skilled in the art. It is, therefore, to be understood that theappended claims are intended to cover all such modifications and changesas fall within the true spirit and scope of the invention.

We claim:

1. in a system for producing high resolution magnified images of aspecimen undergoing observation with a scanning electron beam, thecombination comprising:

magnetic field generating means for sweeping the electron beam in apredetermined two-dimensional pattern over the surface of the specimenundergoing observation; means coupled to said magnetic field generatingmeans for controlling the rate at which said electron beam is swept ineach direction over the surface of said specimen; said rate controllingmeans including crystal oscillator means; counter means coupled to saidoscillator means for producing an output pulse after a predeterminednumber of oscillator pulses have transpired; sweep generating meanscoupling said counter means to said magnetic field generating means soas to drive the electron beam smoothly in one of the two dimensions; andcircuit means coupling said counter means to said magnetic fieldgenerating means so as to drive the electron beam incrementally in theother of the two dimensions after said predetermined number ofoscillator pulses have transpired;

high gain amplifier means;

conductive means connecting said specimen to said amplifier means, saidamplifier means being responsive to elec trons leaving said specimenthrough said conductive means; optical display means having an imagedisplay surfacemeans coupling said magnetic field generating means tosaid optical display means for sweeping said pattern over the surface ofsaid display means in synchronism with the pattern swept by saidelectron beam; and

means coupling said amplifier means to said optical display means formodulating the intensity of said pattern as said pattern is traced outover the surface of said display means. 2. The system of claim 1 whereinsaid magnetic field generating means includes an operational amplifier,an electron beam deflection coil having one of two terminals coupled tothe output of said operational amplifier, and feedback resistance meanscoupling the other of said two terminals to the input of saidoperational amplifier.

3. The system of claim 2 wherein said optical display means comprises acathode ray tube and wherein said means coupling said magnetic fieldgenerating means to said optical display means includes an additionaloperational amplifier, an additional electron beam deflection coilpositioned to deflect the cathode ray tube electron beam and having oneend thereof connected to the output of said additional operationalamplifier, and an operational amplifier feedback resistor con nectingthe input of said additional operational amplifier to the other end ofsaid additional electron beam deflection coil. 4. The system of claim 1wherein said conductive means includes a specimen holder and meansconnecting said specimen holder directly to the input of said high gainamplifier means so as to furnish the electrons leaving said specimen tothe input of said high gain amplifier means.

5. in a system for producing high resolution magnified images of aspecimen undergoing observation with a scanning electron beam, thecombination comprising:

magnetic field generating means for sweeping the electron beam in apredetermined two-dimensional pattern over the surface of the specimenundergoing observation;

means coupled to said magnetic field generating means for controllingthe rate at which said electron beam is swept in each direction over thesurface of said specimen;

high gain amplifier means including first and second operationalamplifiers, said second operational amplifier having and second inputmeans for accepting signals of ing:

positive and negative polarity respectively, and switching meanscoupling the output of said first operational amplifier selectively tothe first or second input of said second operational amplifier inaccordance with the polarity of image to be displayed on said opticaldisplay means;

conductive means connecting said specimen to said amplifier means, saidamplifier means being responsive to electrons leaving said specimenthrough said conductive means;

optical display means having an image display surface;

means coupling said magnetic field generating means to said opticaldisplay means for sweeping said pattern over the surface of said displaymeans in synchronism with the pattern swept by said electron beam; and

means coupling said amplifier means to said optical display means formodulating the intensity of said pattern as said pattern is traced outover the surface of said display means.

6. A high resolution scanning electron microscope comprismeanssupporting a specimen to be examined within an evacuated chamber;electron beam generating means situated within said evacuated chamberand positioned to direct a beam of electrons toward said means forsupporting said specimen;

magnetic field generating means for sweeping said beam of electrons in araster pattern over said means for supporting said specimen; saidmagnetic field generating means including an operational amplifier, anelectron beam deflection coil having one of two terminals coupled to theoutput of said operational amplifier, and feedback resistance meanscoupling the other of said two terminals to the input of saidoperational amplifier;

means coupled to said magnetic field generating means for controllingthe rate at which said electron beams is swept over the surface of saidspecimen;

high gain amplifier means;

conductive means connecting said means supporting said specimen to theinput of said high gain amplifier means; cathode ray tube display means;means coupling said magnetic field generating means to said cathode raytube display means for sweeping said raster pattern over the face ofsaid cathode ray tube in synchronism with the raster pattern swept bysaid beam of electrons; and a means coupling said amplifier means tosaid cathode ray tube display means for modulating the intensity of theraster pattern being traced out over the face of said cathode ray tubedisplay means;

7. The high resolution scanning electron microscope of claim 6 whereinsaid amplifier means includes first and second operational amplifiers,said second operational amplifier having first and second input meansfor accepting signals of positive and negative polarity respectively,and switching means coupling the output of said first operationalamplifier selectively to one of the inputs of said second operationalamplifier so as to selectively produce a positive or negative image onthe face of said cathode ray tube display means.

8. The high resolution scanning electron microscope of claim 6 whereinsaid means coupling said magnetic field generating means to said cathoderay tube display means includes an additional operational amplifier, anadditional electron beam deflection coil positioned to deflect thecathode ray tube electron beam and having one end thereof connected tothe output of said additional operational amplifier, and an additionaloperational amplifier feedback resistor connecting the input of saidadditional operational amplifier to the other end of said additionalelectron beam deflection coil.

9. The high resolution scanning electron microscope of claim 8 includingmeans for controllably tilting the means supporting said specimen so asto permit said beam of electrons to impinge upon said specimen at eitherof two predetermined angles of incidence.

